Triphyenylmethane dyes are known and it has been established that transition metal oxides like dichromate or manganese dioxide will convert the corresponding leuco compound to the triphenylmethane dye. However, large amounts of transition metal salts are produced in the dyes. The dyes have to be salted out of the aqueous solution with ammonium, alkali metal or alkaline earth metal chlorids or sulfates. The resulting dyes contained therefore more or less amounts of salts.
The U.S. Pat. No. 4,566,999 discloses a process for the preparation of acid dyestuffs of low electrolyte content by the oxidation of the leuco compound with manganese dioxide in the presence of phosphoric acid and ammonia.
An object of the present invention is to provide a process for the preparation of triphenylmethane dyes, which are substantially free of heavy metal residues.